This invention relates to industrial filtration, and more particularly, to a method for rendering a PVDF filter membrane hydrophilic.
Microfiltration is used in the pharmaceutical, biotechnology, microelectronics, chemical and food industries to ensure product and process purity. In such applications, microporous membranes serve as filter media. These microporous membranes are formed from inert plastic materials. Typically, the pore size of the membrane is in the order of 0.1 to 5.0 microns, depending upon the particle size of the impurity. A typical microfilter used in industrial applications comprises a replaceable filter cartridge mounted in a housing, which has input and output ports in the process flow path. Such a filter cartridge has a pleated membrane arranged in a cylindrical configuration.
It is important that the filter be inert and produces no extractables that can corrupt the fluid being filtered. Polyvinyldifluoride (PVDF) can be formed into microporous sheets that have the described characteristics. Unfortunately, PVDF is hydrophobic. In order to use PVDF as a filter medium in a water based process, the PVDF must be rendered hydrophilic.
According to the invention, a microporous PVDF membrane is rendered hydrophilic without making it brittle, lowering its heat resistance during sterilization, or producing extractables by exposing the membrane to plasma under controlled conditions. Specifically, a microporous PVDF membrane is first coated with an appropriate hydrophilic monomer. After the monomer dries, the membrane is exposed to the plasma to polymerize the monomer and form a hydrophilic surface on the membrane. Any remaining monomer on the membrane is removed. Thereafter, the membrane is preferably pleated and incorporated in a filter cartridge.
A feature of the invention is the use of particular electrode configuration to expose the coated membrane to a plasma. Specifically, the electrodes lie in a vertical plane adjacent and parallel to the horizontal transport path of the membrane and outside the space between the electrodes. As a result, the membrane is only indirectly exposed to the plasma generated by the electrodes during transport.